Illumination systems are used as either stand-alone light sources or as internal light sources for more complex optical systems. Examples of optical systems that utilize or incorporate illumination systems include projection displays, flat-panel displays, avionics displays, automotive lighting, residential lighting, commercial lighting and industrial lighting applications.
Many applications require illumination systems with high luminance (brightness) and a small effective emitting area. The term “luminance” or brightness is defined as lumens per unit area per unit solid angle. An example of a conventional light source with high luminance and a small effective emitting area is an arc lamp source, such as a xenon arc lamp or a mercury arc lamp. Arc lamp sources may have emitting areas as small as a few square millimeters. An example of a complex optical system that can utilize an illumination system with high luminance and a small effective source area is a projection television display. Current projection television displays typically project the combined images of three small red, green and blue cathode-ray-tube (CRT) devices onto a viewing screen using a projection lens. More recent designs sometimes use a small-area arc lamp source to project images from a liquid crystal display (LCD), a liquid-crystal-on-silicon (LCOS) device or a digital light processor (DLP) device onto a viewing screen. Light emitting diode (LED) sources are currently not used for projection television displays because LED sources do not have sufficient output luminance.
In a conventional optical system that transports light from an input source at one location to an output image at a second location, one cannot produce an optical output image that has a luminance higher than the luminance of the light source. A conventional optical system 10 of the prior art is illustrated in FIG. 1A. In FIG. 1A, light rays 18 from an input light source 12 are focused to an output image 16 using a convex lens 14. The conventional optical system 10 in FIG. 1A can also be illustrated in a different manner as optical system 20 in FIG. 1B. In FIG. 1B, the input source 22 has area, Areain. The light from input source 22 fills a cone 23 covering a solid angle 27, which is shown in cross-section in FIG. 1B. The magnitude of solid angle 27 is Ωin. Lens 24 focuses the light to image 26 having area, Areaout. The light forming the image 26 fills a cone 25 covering a solid angle 28, which is shown in cross-section. The magnitude of solid angle 28 is Ωout. If the optical system has no losses, the light input flux at the input source 22, φin=(Lin)(Areain)(Ωin), equals the light output flux at the output image 26, φout=(Lout)(Areaout)(Ωout). In these equations, Lin is the luminance of the input source 22, Lout is the luminance of the output image 26, “Areain” is the area of the input source 22 and “Areaout” is the area of the output image 26. The quantities Ωin and Ωout are, respectively, the solid angles subtended by the input source and output image light cones. In such a lossless system, it can be shown that Lin=Lout and (Areain)(Ωin)=(Areaout)(Ωout). If the index of refraction of the optical transmission medium is different at the input source and output image positions, the equality (Areain)(Ωin)=(Areaout)(Ωout) is modified to become (nin2)(Areain)(Ωin)=(nout2)(Areaout)(Ωout), where nin is the index of refraction at the input position and nout is the index of refraction at the output position. The quantity (n2)(Area)(Ω) is variously called the “etendue” or “optical extent” or “throughput” of the optical system. In a conventional lossless optical system, the quantity (n2)(Area)(Ω) is conserved.
In U.S. Pat. No. 6,144,536, Zimmerman et al demonstrated that for the special case of a source that has a reflecting emitting surface, an optical system can be designed that recycles a portion of the light emitted by the source back to the source and transmits the remainder of the light to an output position. Under certain conditions utilizing such light recycling, the effective luminance of the source as well as the output luminance of the optical system can be higher than the intrinsic luminance of the source in the absence of recycling, a result that is not predicted by the standard etendue equations. An example of a light source with a reflecting emitting surface is a fluorescent lamp. In FIG. 2A is shown a cross-section 30 of a fluorescent lamp. The lamp has a glass envelope 32 enclosing a hollow interior 36. The hollow interior 36 is filled with a gas that can emit ultraviolet light when a high voltage is applied. The ultraviolet light excites a phosphor coating 34 on the inside surface of the glass envelope, causing the phosphor to emit visible light. The phosphor coating 34 is a partially reflecting surface in addition to being a light emitter. Therefore, it is possible to design an optical system that recycles a portion of the light generated by the phosphor coating 34 back to the coating 34 and thereby cause an increase in the effective brightness of the fluorescent lamp.
The disclosures on light recycling in U.S. Pat. No. 6,144,536 relate to linear light sources that have long narrow emitting apertures with aperture areas greater than 100 mm2. Such configurations, which typically use fluorescent lamps as light sources, are not suitable for many applications such as illumination systems for large projection displays. At the time of the application for U.S. Pat. No. 6,144,536, a typical LED had an output of only 1 lumen per square millimeter and a light reflectivity of less than 20%. To make an illumination system for a projection television that needs 1000 lumens would require at least 1000 LEDs having a total surface area of 1000 mm2. If 1000 such low-reflectivity, low-output LEDs were placed in a brightness-enhancing optical cavity having an output aperture with an area of 10 mm2, the overall output efficiency would be much less than 1%. In other words, less than 10 lumens from the original 1000 lumens would exit the cavity. Such an illumination system would be neither useful nor practical.
Recently, highly reflective green and blue LEDs based on gallium nitride (GaN) semiconductor materials have been developed. Some of these devices have high light output, high luminance and have a light reflecting surface that can reflect at least 50% of the light incident upon the device. Luminance outputs up to several megacandelas per meter squared and total outputs exceeding a hundred lumens from a single packaged device are possible. Light outputs per unit area can exceed 30 lumens/mm2. As such, several new applications relating to illumination systems have become possible. Advantages such as spectral purity, reduced heat, and switching speed all provide motivation to further the use of LEDs, replacing fluorescent, incandescent and arc lamp sources. FIG. 2B shows a recently developed type of LED 40 that has an emitting volume 46 located below both a transparent top electrode 43 and a second transparent layer 44. Light rays 45 are emitted by emitting volume 46 when an electric current is passed through the device 40. Below the emitting volume 46 is a reflective layer 47 that also serves as a portion of the bottom electrode. Electrical contacts 41 and 42 provide a pathway for electrical current to flow through the device 40. It is a recent new concept to have both electrical contacts 41 and 42 on the backside of the LED opposite the emitting surface. Typical prior LED designs placed one electrode on top of the device, which interfered with the light output from the top surface and resulted in devices with low reflectivity. The reflecting layer 47 allows the LED to be both an emitter and a reflector. Lumileds Lighting LLC, for example, produces highly reflective green and blue LED devices of this type. It is expected that highly reflective red LEDs with high outputs and luminance with also eventually be developed. However, even the new green and blue gallium nitride LEDs do not have sufficient luminance for many applications such as large projection television displays.
It would be highly desirable to develop LED-based illumination systems utilizing light recycling that have both a small effective emitting area and sufficient brightness for applications such as projection displays, flat-panel displays, avionics displays, automotive lighting, residential lighting, commercial lighting and industrial lighting applications.